Sig M400 barrel - gas port size

[davsel]

I recently built an AR upper using an M400 Style barrel from Top Notch Top Ends (TNTE): GREEN MOUNTAIN 1/7 16" 4150 SIG M-400 STYLE BARREL CHROME LINED (http://www.tntesales.com/green-mountain-1-7-16-4150-sig-m-400-style-barrel-chrome-lined/).
I've used this barrel before with great success - very accurate.

When I test fired this build, it short-stroked every round.
I swapped lowers - same
BCGs - same
Gas block/tube - same

I figured it must be the gas port. I placed my smallest allen wrench in the port of an M4 and M400 and the M400 is definitly smaller - less wiggle room with wrench.

I'm guessing TNTE sent me a Sig516 (piston) barrel instead of a SigM400 (gas) barrel - And the barrel is also16.5" instead of the advertised 16"

I can't send it back due to having already mounted and fired it, but at least it is an undersized port instead of oversized and I can fix it.

What does it take to drill out the gas port to the proper dimension? Is it a DIY? Cost?




Note: I will never deal with TNTE again. After waiting almost 6 weeks for a barrel I assumed they had in stock, having (all) 3 emails and (all) 2 voicemails ignored, and reading through the fine print on the TNTE site, I searched and found this: BBB Business Review (http://www.bbb.org/wisconsin/business-reviews/ammunition/top-notch-top-ends-in-darien-wi-1000015057/)

[BPTactical]

Gas Port size for a 16" 5.56 barrel should be in the .063-.078 range iirc.
Drilling the out is possible but a smidge tricky or you run the risk of breaking a bit in the port.
Use a new, good quality bit.
The port must be dead nuts vertical relative to the drill. Drill it in one shot, no drill a bit, lift up and check etc. The steel will work harden and be a bitch.
Slide a brass rod in the bore to keep from dinging the barrel.

[MarkCO]

I would not recommend drilling out the gas port on a chrome-lined barrel. Gas port erosion is bad enough without giving it a head start.

Gas port is but one parameter in the system. You can drop spring rate, or mass from the buffer to tune the system to run with the gas port you have. If you had a barrel that did not have an interior lining, sure, drilling out the gas port is one way to tune the rifle.

[davsel]

Well damn.
I don't want to have a one-off buffer/spring/BCG in the arsenal.

Anyone want to trade a good Carbine barrel for a Sig516 barrrel with 10 rounds down it?

Thanks for the inputs.

[davsel]

After TNTE insisted it is the same barrel as I previously purchased in 2013, and a "piss-off, you bought it, you deal with it" customer service attitude, TNTE tells me to send the barrel back to Green Mountain to have them check it out.
Green Mountain won't even tell me if the barrel specs have changed since Feb 2013 - just send it to us to check it out.

However, I've already ordered a $50 spiral fluted carbide #50 reamer to open the port to 0.070.
Hopefully I'll be able to test fire it this weekend.

Buyer-Beware - Top Notch Top Ends